In
League of Legends, you can hold up to 6 items in your champion's item slots
To make the best use of them, you should try filling up all slots at
all times.

This is because there are some items that are very gold efficient
but not
as slot efficient.
These are usually the cheap items such as and having them will
greatly boost your early game power.Therefore, a
player who has empty item slots will lose out on their early advantage
by not purchasing these items

But
do keep in mind that all Doran's items and Potions have only 40%
resell value so be mindful about buying too many of them

On the other hand, the following items have resell value of 70%

An Example
of a Good ADC Build Order(Go over these
slowly to see which items are being replaced)

Doran's Items
indeed have more powerful stats than basic items (but not so much
anymore).
Therefore, even if you would lose some money by selling Doran's Items,
you should still get them.

4. Special
Restrictions

If you are going to purchase , do
not buy (since this is just another scaling mp regen item). This is because a champion who is mana
hungry enough to buy buys
mana items at a discount, making 154 mana from rather inefficient.
As a result loses its gold efficiency
and it will be only good as .
It also interferes with getting early
for mana stacking.

is currently an
underpowered item which should be avoided in most
cases. It should be only purchased if you are laning agaisnt
ranged AP champions who harass a lot with autoattacks such as Lulu,
Kayle, and Teemo. Otherwise is
outclassed by most of the basic items

is
also an inferior item to melee champions especially for those that
do not stack AD because they do not autoattack minions as much.
It is only
viable on ranged ADC.

You have now learned how
to manage the item slots
to optimize their usage